Unlike the capillaries in other tissues such as muscles, the capillaries that supply the blood to most of the brain tissues except some areas including the circumventricular organs (pineal gland, pituitary body, area postrema, etc.) differ in that the endothelial cells forming their endothelium are mutually connected by tight intercellular junctions. Passive transfer of substances from the capillaries to the brain is thereby restricted, and although there are some exceptions, substances are unlikely to move into the brain from the blood except such compounds as are lipid-soluble or of low-molecular-weight (less than 200-500 Dalton) and electrically neutral around the physiological pH. This system, which restricts exchange of substances between the blood and the tissue fluid of the brain through the endothelium of capillaries in the brain, is called the blood-brain barrier or BBB. The blood-brain barrier not only restricts exchange of substances between the blood and the brain but also between the tissue fluid of the central nervous system, including the brain and the spine, and the blood.
Owing to the blood-brain barrier, most of the cells of the central nervous system escape the effects of fluctuating concentrations of substances like hormones and lymphokines in the blood, and their biochemical homeostasis is thus maintained.
The blood-brain barrier, however, imposes a problem when it comes to develop pharmaceutical agents. For mucopolysaccharidosis type I (Hurler syndrome), an inherited metabolic disease caused by α-L-iduronidase deficiency, for example, although an enzyme replacement therapy is carried out by intravenous supplementation with a recombinant α-L-iduronidase as a therapy, the therapy is not effective for the notable abnormality observed in the central nervous system (CNS) in Hurler syndrome because the enzyme cannot pass through the blood-brain barrier.
Development of various methods has been attempted to make those macromolecular substances as proteins or the like, which need to be brought into function in the central nervous system, pass through the blood-brain barrier. In the case of nerve growth factor, for example, while attempts have been made for a method to cause the factor to pass through the blood-brain barrier by allowing liposomes encapsulating the factor to fuse with the cell membrane of endothelial cells in brain capillaries, they have not been materialized (Non Patent Literature 1). In the case of α-L-iduronidase, an attempt has been made to enhance the passive transfer of the enzyme through the blood-brain barrier by raising its blood concentration through an increased single dose of the enzyme, and it thus has been demonstrated, using a Hurler syndrome animal model, that the abnormality in the central nervous system (CNS) is ameliorated by that method (Non Patent Literature 2).
Furthermore, circumventing the blood-brain barrier, an attempt has also been made to administer a macromolecular substance directly into the medullary cavity or into the brain. For example, reports have been made about a method in which human α-L-iduronidase was administered into the medullary cavity of a patient with a Hurler syndrome (mucopolysaccharidosis type I) (Patent Literature 1), a method in which human acid sphingomyelinase was administered into the brain ventricles of a patient with Niemann-Pick disease (Patent Literature 2), and a method in which iduronate 2-sulfatase (I2S) was administered into the brain ventricles of Hunter syndrome model animals (Patent Literature 3). While it seems possible by one of such methods to definitely let a pharmaceutical agent act in the central nervous system, they have a problem as being highly invasive.
There have been reported various methods to let a macromolecular substance get into the brain through the blood-brain barrier, in which the macromolecular substance is modified to give it an affinity to membrane proteins occurring on the endothelial cells of the brain capillaries. Examples of those membrane proteins which occur on the endothelial cells of the brain capillaries include receptors for compounds such as insulin, transferrin, insulin-like growth factor (IGF-I, IGF-II), LDL, and leptin.
For example, a technique has been reported in which nerve growth factor (NGF) was synthesized into the form of a fusion protein with insulin, and this fusion protein was allowed to pass through the blood-brain barrier via its binding to the insulin receptor (Patent Literatures 4-6). Further, a technique has been reported in which nerve growth factor (NGF) was synthesized in the form of a fusion protein with anti-insulin receptor antibody, and this fusion protein was allowed to pass through the blood-brain barrier via its binding to the insulin receptor (Patent Literatures 4 and 7). Further, a technique has been reported in which nerve growth factor (NGF) was synthesized in the form of a fusion protein with transferrin, and this fusion protein was allowed to pass through the blood-brain barrier via its binding to the transferrin receptor (TfR) (Patent Literature 8). Further, a technique has been reported in which nerve growth factor (NGF) was synthesized in the form of a fusion protein with anti-transferrin receptor antibody (anti-TfR antibody), and this fusion protein is allowed to pass through the blood-brain barrier via its binding to TfR (Patent Literatures 4 and 9).
Looking further into the techniques that utilize an anti-transferrin receptor antibody, there has been reported that in the field of the technique to make a pharmaceutical agent pass through the blood-brain barrier by binding it to an anti-TfR antibody, a single-chain antibody could be used (Non Patent Literature 3). Further, it has been reported that anti-hTfR antibodies exhibiting relatively high dissociation constants with hTfR (low-affinity anti-hTfR antibody) could be favorably used in the technique to make pharmaceutical agents pass through the blood-brain barrier (Patent Literatures 10 and 11, and Non Patent Literature 4). Still further, it has also been reported that an anti-TfR antibodies whose affinity to hTfR varies depending on pH could be employed as a carrier for making pharmaceutical agents pass through the blood-brain barrier (Patent Literature 12, and Non Patent Literature 5).